Don't scan disk for the value of CommitSyncWrites if present in HashTable

We don't need to scan disk for the on disk committed value if the item is present in the HashTable during a backfill as the item is sufficient to send to the consumer.

Historically we did this because we originally intended on sending Commits from disk as DCP Commit messages to the consumer. This was not possible as the DCP Commit message does not contain the full value and so we decided to send DCP Mutations for CommitSyncWrites when sending them from Disk.